I've got stock gutted downpipes with the M valves ziptied open and I get slight drone when going uphill while the car is trying to be efficient as it's holding 6th gear. Sometimes I'll bump down a gear and it'll go away. I'll be removing my muffler and adding Vibrant 1794 resonators for what they have claimed as a perfect sounding exhaust without any drone.
http://f10.m5post.com/forums/showthread.php?t=1248573
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'10 BMW X5///M - Alpine White on Sakhir Orange - JB4 w/ HCP Stage 2 BEF, NGK 97506 @ .20, SS tubing & BMC filters, gutted cats, AC Forged 312 22's, H&R 2"
'98 BMW 740iL - ///E39 M5 6spd swap, fully built engine (sleeved, P&P, cams), 3.46 LSD, H&R Stage 2/Bilstein HD, Magnaflow 14816's
'97 BMW 328ci - ///E36 M3 clutch & L/W flywheel, Z4///M 18's